Routines
Prev: 6998 Up: Map Next: 69BC
699D: Plot Pixels
Used by the routine at ClearPixels.
Sets or clears a block of pixels, going up from the bottom row and across from the left column. Which of the two depends on the operands that have been written by ClearPixels or SetPixels.
Input
B Height of the block in pixels
C Width of the block in pixels
HL Pixel co-ordinates of the bottom left of the block (H is the Y position and L is the X position)
PlotPixels 699D PUSH BC Stash BC and HL on the stack.
699E PUSH HL
699F LD A,L A=L.
69A0 CPL Flip the bits.
69A1 RLCA A=A * 08.
69A2 RLCA
69A3 RLCA
69A4 AND %00111000 Keep only bits 3-5.
Self modifying code
Altered From Bytes bits
ClearPixels 86 10000110
SetPixels C6 11000110
69A6 OR %11000110 Set bits 1-2, 6-7 (this operand is modified).
69A8 LD ($69AF),A Store A as the opcode of the instruction at 69AF.
69AB CALL ScreenAddress Call ScreenAddress.
PlotPixels_Column 69AE SET 0,(HL) Set (or reset) the bit of the byte at HL for this pixel.
69B0 CALL ScreenPosition1PixelAbove Call ScreenPosition1PixelAbove to move up one pixel.
69B3 DJNZ PlotPixels_Column Decrease the height counter by one and loop back to PlotPixels_Column until the whole column has been set (or cleared).
69B5 POP HL Restore HL and BC from the stack.
69B6 POP BC
69B7 INC L Move on to the next pixel column.
69B8 DEC C Decrease the width counter by one and loop back to PlotPixels until all of the columns have been done.
69B9 JR NZ,PlotPixels
69BB RET Return.
Prev: 6998 Up: Map Next: 69BC